This invention relates to a carriage positioning system and is particularly useful in a system using cables for positioning a carriage along either the X or Y axis.
The invention can be used particularly effectively in reverse axis plotters of the type in which the width dimension of the plotter is greater than its length and in which a tool such as a pen or other instrument is moveable in X and Y coordinate directions relative to a surface supporting a sheet of paper or other sheet material upon which figures, characters or other graphics are created; and deals more particularly with an improvement in the drive system responsible for moving the Y carriage wherein means are provided in the system for more evenly moving the ends of the Y carriage along the length of the plotter to thereby position the drawing instrument along a desired path to be followed with enhanced accuracy.
In the co-pending application now Ser. No. 195,128, now U.S. Pat. No. 4,916,819 a plotter is disclosed wherein two drive systems are employed each respectively driving X and Y carriages for moving a drawing instrument relative to a support surface through X and Y coordinate directions. In each of these drive systems a single cable is driven by a rotating drum and is so looped relative to its associated carriage to move it along a given coordinate axis. Since in these plotters, the Y carriage spans its width and is thus substantially shorter than the length of the plotter, the Y carriage must be moved at its ends by the cable which drives it with particular evenness since the slightest skewing of this carriage may result in a flaw in the graphic information being created by the moving instrument on the sheet material below it.
A problem particular to such single cable Y carriage drives is that as the cable feeds out from one portion of the drum and is taken up by the other, an unequal number of winds are often formed on the drive drum between the lengths of cable which connect to each end of the Y carriage. This unequal winding on the drive drum may undesirably result at times in skewing of the Y carriage due to disproportionate stretching of the cable.
Accordingly it is an object of the present invention to provide a cable drive system in a plotter such as aforesaid which is capable of moving a carriage at its opposite ends without skewing it relative to the direction which it is moved.
It is yet a further object of the present invention to provide a drum construction capable of simultaneously wrapping and feeding out two separate cables each individually connected to the opposite ends of a carriage.
Still a further object of the present invention is to provide a cable drive system using two relatively short, equal length cables to provide a drive system having high spring constants to assure a high frequency response system.